The present invention relates to a method of storing a lanthanum oxide target that degrades easily due to hydroxylation, and also relates to a vacuum-sealed lanthanum oxide target.
Lanthanum as a rare earth metal is contained in the earth's crust as a mixed composite oxide. Rare-earth elements are called “rare-earth” elements because they are separated from relatively rare minerals, but they are not that rare in light of the overall earth's crust. In recent years, rare earth metals are attracting attention as an electronic material, and research and development for using rare earth metals are being promoted.
Among the rare earth metals, lanthanum (La) is attracting particular attention. To briefly introduce lanthanum, lanthanum is a white metal having an atomic number of 57 and an atomic weight of 138.9, and comprises a double hexagonal close-packed structure at normal temperature. Lanthanum has a melting point of 921° C., boiling point of 3500° C., and density of 6.15 g/cm3, its surface is oxidized in the atmosphere, and gradually melts in water.
Lanthanum is soluble in hot water and acid and, although it is not ductile, it is slightly malleable. Lanthanum's resistivity is 5.70×10−6Ω cm, and it becomes oxide (La2O3) when burned at 445° C. or higher (refer to Dictionary of Physics and Chemistry). With rare earth elements, it is generally said that compounds with the oxidation number 3 are stable, and lanthanum is also trivalent.
Lanthanum is a metal that is attracting attention as an electronic material such as a metal gate material or a High-k material. Rare earth elements other than lanthanum also have attributes that are similar to lanthanum.
A rare earth metal such as lanthanum is a material in which high purification is difficult to achieve since it is easily oxidized during the refining process. In addition, if a rare earth metal such as lanthanum is left in the atmosphere, there is a problem in that the handling thereof is difficult since it will become oxidized and discolored in a short period of time.
In recent years, thinning of a gate insulator film in the next-generation MOSFET is being demanded, but with the SiO2 that has been conventionally used as the gate insulator film, the leak current will increase due to the tunnel effect, and normal operation is becoming difficult.
Thus, as a substitute for the SiO2 described above, HfO2, ZrO2, Al2O3 and La2O3 with high dielectric constant, high thermal stability, and high energy barrier against the holes and electrons in the silicon have been proposed. In particular, among the foregoing materials, La2O3 is valued highly, and a research paper that studied its electrical properties and use as a gate insulator film in the next-generation MOSFET has been reported (refer to Non-Patent Document 1). Nevertheless, with Non-Patent Document 1, the subject of research is a La2O3 film, and it does not make any reference to the hygroscopic property and pulverization phenomenon of a lanthanum oxide target.
It could be said that rare earth metals such as lanthanum and their oxides are still on the research phase, but when studying the properties of such rare earth metal and their oxides, if a rare earth metal oxide, in particular if lanthanum oxide itself exists as a sputtering target material, it is possible to form a thin film of such lanthanum oxide on a substrate. It will also be easy to study the behavior at the interface with the silicon substrate, and additionally study the properties of a high-dielectric gate insulator film or the like by forming a rare earth metal compound, and there is also a significant advantage in that the flexibility of the target as a product will increase.
Nevertheless, even if a lanthanum sputtering target is prepared, as described above, it becomes oxidized in a short period of time in the atmosphere. Generally speaking, a stable oxide layer is formed on a metal target surface, but since it is extremely thin under normal circumstances, it peels off during the initial stage of sputtering, and will not affect the sputtering characteristics significantly. However, with a lanthanum sputtering target, the oxide layer becomes thick and deteriorates the electrical conductivity, and thereby causes defective sputtering.
In addition, if the lanthanum sputtering target is left in the atmosphere for a long period of time, it reacts with the moisture in the air and becomes covered with white hydroxide powder, and ultimately becomes degraded, and it may even cause a problem of not allowing normal sputtering to be performed. Thus, after the target is prepared, it is necessary to take measures for preventing oxidation and hydration such as by immediately performing vacuum packing or coating the target with fats and oils.
As a method of storing rare earth metals, the standard method is to store the rare earth metals in mineral oil in order to prevent such rare earth metals from coming in contact with the atmosphere. However, when using a rare earth metal as a sputtering target, it is necessary to clean the target before its use in order to remove the mineral oil. However, there is a problem in that the cleaning of the target itself is difficult due to its reactivity with oxygen, moisture, and carbon dioxide.
Accordingly, under normal circumstances, it is necessary to store and pack the target based on vacuum packing. However, even in a state where the target is vacuum packed, since pulverization caused by hydroxylation will progress even with a small amount of moisture that permeates the film to be used, it was difficult to store the sputtering target for a long period of time in a usable condition.
When reviewing the conventional background art, there are the following methods; namely, a method of covering the hollow cathode-type sputtering target with a resin bag (refer to Patent Document 1), a method of affixing a plastic protective film to the target (refer to Patent Document 2), a method of packaging the target using a film having a surface that is free from detachable particles (refer to Patent Document 3), a method of using a transparent acrylic resin cover to prepare a target storage container and screwing shut the storage container (refer to Patent Document 4), and a method of filling the sputtering target in a bag-shaped object (refer to Patent Document 5). Nevertheless, since the foregoing Documents seal the target using a resin cover or a resin film, they are insufficient as a method of storing a target made of lanthanum oxide.
Moreover, there is a report that hydration can be inhibited by placing the lanthanum oxide powder in a hydrofluoric acid aqueous solution and forming a lanthanum fluoride film on the powder surface (refer to Patent Document 6). Although this is of some help, since its object is lanthanum oxide powder, it is unclear as to whether it can be applied to a target of bulk shape or block shape.
In light of the foregoing circumstances, the present applicant developed a method of storing a sputtering target made of lanthanum oxide, wherein the lanthanum oxide target and lanthanum oxide powder are charged in a vacuum pack with an oxygen transmission rate of 0.1 cm3/m2 per 24 h at 1 atm or less and a moisture vapor transmission rate of 0.1 g/m2 per 24 h or less, and, after charging the target and powder, subjecting the vacuum pack to vacuum suction and sealing for storage (refer to Patent Document 7).
This storage method is extremely effective and yields a dramatically superior effect, in comparison to conventional technologies, of being able to inhibit the pulverization phenomenon caused by hydration (hydroxylation), and the pulverization phenomenon caused by the formation of carbonate. Nevertheless, it is necessary to further improve this storage method.
[Patent Document 1] Publication Number of WIPO: WO2005/037649
[Patent Document 2] Japanese Published Unexamined Application No. 2002-212718
[Patent Document 3] Japanese Published Unexamined Application No. 2001-240959
[Patent Document 4] Japanese Published Unexamined Application No. H8-246135
[Patent Document 5] Japanese Published Unexamined Application No. H4-231461
[Patent Document 6] Japanese Published Unexamined Application No. H10-87326
[Patent Document 7] Publication Number of WIPO: WO2010/050409
[Non Patent Document 1] Written by Eisuke Tokunaga and two others, “Research on Oxide Material for High-k Gate Insulator Film” The Institute of Electrical Engineers of Japan, Research Paper of Electronic Materials, Vol. 6-13, Pages 37 to 41, Published on Sep. 21, 2001